Definite and Indeinite Articles - معرفة ونكرة - (marifah wa nakirah)
In English:
A book - this is called indefinite article
The book - this is called definite article
In Arabic:
Most of the indefinite articles are indicated by a tanween.
for example, كِتَابٌ - kitabun. This means 'a book'.
the taween at the end indicates that it is an indefinite article.
To make this a definite article, we just need to add al (ال) in the begining and chnage the tanween to single harakah.